In a typical thermal printer, a web-type dye-carrier containing a series of spaced frames of different colored heat transferable dyes is spooled on a carrier supply spool. The carrier is paid out from the supply spool and rewound on a take up spool. The carrier moves through a nip formed between a thermal print head and a dye-absorbing receiver sheet. The receiver sheet is clamped to a rotatable drum. The receiver sheet may, for example, be coated paper and the print head is formed of a plurality of heating elements. When heat is supplied to the dye-carrier, dye is transferred to the receiver sheet.
At the beginning of a print cycle, the receiver sheet must be clamped to the drum which is at a home position. After being clamped to the drum, the receiver is advanced under the print head. The heating elements of the print head are energized to form a dye image. The drum makes several revolutions as different colored dye images are applied into the receiver. In this way, a final, full-colored image is produced. After this full-colored image is produced, the direction of the drum is reversed, and when a position is reached the clamp is opened and the receiver sheet is ejected from the thermal printer. The drum then advances the clamping mechanism back to the home position and the above process is repeated.
The process of clamping the receiver sheet to the drum must be done in a manner that prevents the receiver sheet from moving during dye transfer. Movement of as little as 0.010 inch can be detected and reduces the image sharpness of the full color image.
Often the clamping mechanism engages a large border portion of the leading edge of the receiver. This border portion is a non-printable area since it is disposed underneath the clamp.